NIST library compound scoring GC-MS
Libraries in MSWS
There are a variety of libraries available for different applications. SCION Instruments offers various libraries including NIST, Wiley and Pfleger/Maurer/Weber (PMW) with customisable libraries and the ability to automatically search in multiple libraries. The most commonly used library is NIST. For the purpose of this technical note, NIST will be used as the example but the principles off all libraries is the same, the main difference is that the compounds available varies between libraries.
National Institute of Standards and Technology (NIST) Library
The National Institute of Standards and Technology (NIST) is one of the oldest physical science laboratories. The organization collected Electron Ionization (EI) Gas Chromatography (GC) mass spectral data of known standards from various sources to create a mass spectral reference library of compounds. The NIST library is used all over the world for the identification of unknown compounds from GC-MS chromatograms. SCION Instruments’ software MS Workstation (MSWS), which is used for analysis using MS, is also compatible with NIST.
Identifying the unknowns
The NIST library will examine the peaks in the chromatogram. The NIST library works as follows; open the chromatogram in the software (MSWS), then click on the peak of interest in the chromatogram, click on the NIST button on top of the data review tab. The NIST library will now open and search for corresponding mass spectra. An example is shown below from the compound acenaphthylene (Figure 1). The red spectrum is from the sample run on the MS and the blue is from acenaphthylene from the NIST library.
Figure 1 NIST mass spectrum comparison
The software calculates several numbers associated with each compound. The three mainly used are: Match Factor (Match),
Reverse Match Factor (R. Match) and Probability (%). A list of different compounds will be displayed and they are ranked based on how well their reference mass spectra matches the unknown spectrum. The compound with the highest spectrum similarity is at top of the list and deemed most likely to be the correct identification of the unknown compound. In this case that is correct (Figure 2).
